Master’s-Level Cell Biology & Advanced Molecular Biology Notes
1. Definition
The PI3K–AKT–mTOR pathway is a major intracellular signaling network that integrates extracellular signals, nutrient availability, cellular energy status and stress to regulate:
- Cell growth
- Cell survival
- Protein synthesis
- Metabolism
- Cell proliferation
- Autophagy
- Lipid and glucose metabolism
The canonical pathway is:
Growth factor → RTK → PI3K → PIP3 → AKT → mTORC1 → protein synthesis/growth
It operates alongside pathways such as RAS–MAPK and frequently cross-talks with them.
2. Core Pathway
GROWTH FACTOR
↓
RTK
↓
PI3K
↓
PIP2 → PIP3
↓
AKT
↓
┌────────┴────────┐
↓ ↓
mTORC1 Other
↓ targets
Protein synthesis
↓
CELL GROWTHCore memory
PI3K → PIP3 → AKT → mTORC1
3. What Is PI3K?
PI3K = Phosphoinositide 3-kinase
PI3Ks are lipid kinases that phosphorylate membrane phosphoinositides.
The most important reaction in the canonical pathway is:
PIP2 → PIP3
More specifically:
PI(4,5)P₂ → PI(3,4,5)P₃
PIP3 functions as a membrane-associated signaling lipid.
4. Major Classes of PI3K
PI3Ks are divided into several classes.
For growth-factor signaling, the most important is:
Class I PI3K
Class I PI3Ks are commonly activated downstream of:
- Receptor tyrosine kinases
- Some GPCRs
- Other receptor-associated signaling proteins
Class IA PI3Ks typically contain:
- Regulatory p85 subunit
- Catalytic p110 subunit
Important catalytic isoforms include:
- p110α
- p110β
- p110δ
Class IB is principally associated with p110γ.
5. Step 1 — Receptor Activation
A growth factor binds an appropriate receptor.
For example:
Growth factor
↓
RTK
↓
Tyrosine phosphorylationActivated receptor proteins provide docking sites for signaling proteins.
6. Step 2 — PI3K Recruitment and Activation
PI3K is recruited to the activated receptor or receptor-associated signaling complex.
RTK-P
↓
PI3K
↓
PI3K activationActivated PI3K then acts on membrane phosphoinositides.
7. Step 3 — PIP3 Generation
PI3K phosphorylates PIP2 at the 3-position of the inositol ring.
PIP2
│
│ PI3K
↓
PIP3PIP3
Phosphatidylinositol 3,4,5-trisphosphate
PIP3 is not simply a metabolic intermediate.
It acts as a second messenger that recruits proteins containing PH domains to the plasma membrane.
8. PH Domains
PH = Pleckstrin Homology
Important signaling proteins containing PH domains include:
- AKT
- PDK1
PIP3 provides a membrane docking site for these proteins.
Plasma membrane
────────────────────────
PIP3
↑
│
PH domain
│
AKTThis spatial organization is essential for efficient AKT activation.
9. AKT
AKT is also called:
Protein kinase B (PKB)
There are three mammalian AKT isoforms:
- AKT1
- AKT2
- AKT3
AKT is a serine/threonine protein kinase.
It is a major mediator of:
- Cell survival
- Growth
- Metabolism
- Protein synthesis
10. AKT Activation
AKT activation requires membrane recruitment and phosphorylation.
Two important upstream kinases are:
PDK1
and
mTORC2
Simplified:
PIP3
↓
AKT recruitment
↓
PDK1 + mTORC2
↓
AKT phosphorylation
↓
ACTIVE AKT11. PDK1
PDK1 = 3-phosphoinositide-dependent protein kinase-1
PIP3 recruits both AKT and PDK1 to the membrane.
PDK1 phosphorylates an important activation-site residue on AKT.
12. mTORC2
mTORC2 = mechanistic target of rapamycin complex 2
mTORC2 contributes to full AKT activation through phosphorylation of AKT at a distinct regulatory site.
Therefore:
PIP3
↓
AKT membrane recruitment
↓
PDK1 + mTORC2
↓
AKT activation13. The Two Major mTOR Complexes
mTOR exists primarily in two functionally distinct complexes:
mTORC1
and
mTORC2
mTOR
│
┌───────┴───────┐
↓ ↓
mTORC1 mTORC2
↓ ↓
Growth/protein AKT regulation
synthesis cytoskeleton14. mTORC1
mTORC1 is the major regulator of:
- Protein synthesis
- Cell growth
- Ribosome biogenesis
- Lipid synthesis
- Nucleotide synthesis
- Autophagy suppression
mTORC1 is highly sensitive to:
- Growth factors
- Amino acids
- Cellular energy
- Oxygen
- Cellular stress
15. mTORC2
mTORC2 regulates:
- AKT activation
- Cytoskeletal organization
- Cell survival
- Cell metabolism
- Other AGC-family kinases
It is particularly important in the spatial and structural regulation of cells.
16. AKT → mTORC1
One of the major functions of AKT is activation of mTORC1.
A simplified mechanism involves inhibition of:
TSC1/TSC2 complex
AKT
↓
TSC1/TSC2 inhibition
↓
RHEB-GTP increases
↓
mTORC1 activation17. TSC Complex
The TSC1–TSC2 complex is an important negative regulator of mTORC1.
TSC2 has GAP activity toward:
RHEB
RHEB is a small GTPase that activates mTORC1 when GTP-bound.
RHEB-GTP
↓
mTORC1 activationTSC activity promotes:
RHEB-GTP → RHEB-GDP
Therefore TSC acts as a brake on mTORC1.
18. AKT Relieves the TSC Brake
Growth factor
↓
PI3K
↓
PIP3
↓
AKT
↓
TSC inhibition
↓
RHEB-GTP
↓
mTORC1This provides an important molecular link between growth-factor signaling and cellular growth.
19. mTORC1 and Protein Synthesis
mTORC1 stimulates protein synthesis mainly through:
- S6 kinase
- 4E-BP1
mTORC1
├────────→ S6K
│ ↓
│ Translation
│
└────────→ 4E-BP1
↓
eIF4E release
↓
Translation20. S6K
S6K = Ribosomal protein S6 kinase
mTORC1 activates S6K.
S6K promotes processes associated with:
- Translation
- Ribosome biogenesis
- Cellular growth
Thus:
mTORC1 → S6K → increased protein synthesis
21. 4E-BP1
4E-BP1 = Eukaryotic translation initiation factor 4E-binding protein 1
In its hypophosphorylated state, 4E-BP1 binds and inhibits:
eIF4E
When mTORC1 phosphorylates 4E-BP1:
4E-BP1-P
↓
eIF4E released
↓
Translation initiationThis promotes cap-dependent translation.
22. Overall Effect on Protein Synthesis
Growth factor
↓
PI3K
↓
AKT
↓
mTORC1
↓
┌────┴─────┐
↓ ↓
S6K 4E-BP1
↓ ↓
Translation initiation
↓
Protein synthesis
↓
Cell growth23. AKT and Cell Survival
AKT is strongly pro-survival.
One mechanism involves inhibition of pro-apoptotic proteins.
AKT can phosphorylate and inhibit members of the FOXO transcription-factor family and influence other apoptotic regulators.
AKT
↓
FOXO inhibition
↓
Reduced expression of some pro-apoptotic genes
↓
Cell survivalThe exact outcome depends on cell type and signaling context.
24. AKT–FOXO Axis
FOXO transcription factors can promote expression of genes involved in:
- Stress resistance
- Cell-cycle arrest
- Apoptosis
- Metabolism
AKT phosphorylation promotes FOXO exclusion from the nucleus.
AKT activation
↓
FOXO phosphorylation
↓
FOXO nuclear exclusion
↓
Altered gene transcription25. AKT and BAD
AKT can phosphorylate the pro-apoptotic protein:
BAD
Phosphorylated BAD is functionally inhibited through interactions with regulatory proteins.
Simplified:
AKT
↓
BAD phosphorylation
↓
Reduced pro-apoptotic activity
↓
Cell survival26. PI3K–AKT and Metabolism
The pathway is particularly important in metabolic regulation.
AKT can influence:
- Glucose uptake
- Glycogen synthesis
- Glucose metabolism
- Lipid metabolism
- Protein synthesis
This is particularly important downstream of:
Insulin receptor signaling
27. Insulin Signaling
A simplified pathway:
INSULIN
↓
Insulin receptor
↓
IRS proteins
↓
PI3K
↓
PIP3
↓
AKT
↓
Metabolic effectsThus PI3K–AKT is central to insulin-mediated cellular responses.
28. GLUT4 Translocation
In insulin-responsive tissues, AKT signaling contributes to translocation of:
GLUT4
to the plasma membrane.
Insulin
↓
PI3K
↓
AKT
↓
GLUT4 vesicle trafficking
↓
GLUT4 at plasma membrane
↓
↑ Glucose uptakeThis is especially important in skeletal muscle and adipose tissue.
29. PTEN — The Major Brake
A key negative regulator of PI3K signaling is:
PTEN
PTEN = Phosphatase and tensin homolog
PTEN dephosphorylates PIP3 and converts it toward PIP2.
PIP3
↓
PTEN
↓
PIP2Thus:
PI3K increases PIP3; PTEN decreases PIP3.
30. PI3K vs PTEN
PIP2
│
│ PI3K
↓
PIP3
│
│ PTEN
↓
PIP2This opposing enzymatic activity provides tight control of AKT signaling.
31. Why PTEN Is a Tumor Suppressor
Loss of PTEN causes excessive PIP3 accumulation.
PTEN loss
↓
↑ PIP3
↓
↑ AKT
↓
↑ mTOR signaling
↓
↑ Growth / survivalTherefore PTEN is an important tumor suppressor.
32. mTOR and Nutrient Sensing
A major modern concept is that mTORC1 does not simply respond to growth factors.
It integrates:
- Amino acids
- Energy
- Growth factors
- Oxygen
- Cellular stress
mTORC1
/ | \
Growth Nutrients Energy
factorsThis allows cells to grow only when environmental conditions are favorable.
33. Amino-Acid Regulation
Amino acids, particularly branched-chain amino acids and arginine, contribute to mTORC1 activation through Rag GTPase-dependent mechanisms and lysosomal recruitment.
Conceptually:
Amino acids
↓
Rag GTPases
↓
mTORC1 lysosomal recruitment
↓
mTORC1 activationThis is distinct from the AKT-dependent growth-factor pathway.
34. Lysosomes as Signaling Platforms
Modern cell biology emphasizes the lysosome as an important signaling platform for mTORC1.
Amino acids
↓
Rag GTPases
↓
Lysosome
↓
mTORC1Thus lysosomes function not merely as degradative organelles but also as metabolic signaling hubs.
35. Cellular Energy and AMPK
Low cellular energy activates:
AMPK
AMP-activated protein kinase
AMPK generally inhibits mTORC1 under energy stress.
Low ATP
↓
AMP/ADP ↑
↓
AMPK activation
↓
mTORC1 inhibition
↓
Reduced anabolic growthThis prevents cells from consuming energy on growth when energy supplies are inadequate.
36. Energy–Growth Integration
CELLULAR CONDITIONS
│
┌─────────────┼─────────────┐
↓ ↓ ↓
Growth factors Nutrients Energy
↓ ↓ ↓
AKT Rag GTPases AMPK
↓ ↓ ↓
└─────────────┼─────────────┘
↓
mTORC1
↓
Cell growth37. mTORC1 and Autophagy
mTORC1 is a major inhibitor of autophagy under nutrient-rich conditions.
Nutrients abundant
↓
mTORC1 active
↓
Autophagy inhibitedWhen nutrients or growth signals are low:
mTORC1 inhibited
↓
Autophagy increases
↓
Macromolecule recyclingThis allows cells to adapt to nutrient deprivation.
38. mTORC1 and Anabolism
mTORC1 promotes anabolic processes including:
- Protein synthesis
- Lipid synthesis
- Nucleotide synthesis
- Ribosome biogenesis
Therefore:
mTORC1 = major anabolic growth regulator
39. mTORC1 and Catabolism
When mTORC1 is suppressed, cells can increase catabolic processes such as autophagy.
High nutrients
↓
mTORC1 ↑
↓
Anabolism ↑
Autophagy ↓
Low nutrients
↓
mTORC1 ↓
↓
Anabolism ↓
Autophagy ↑40. mTORC2
mTORC2 is less directly sensitive to acute nutrient regulation than mTORC1 and is involved in:
- AKT activation
- Cytoskeletal organization
- Cell survival
- Metabolism
- Regulation of other AGC kinases
Its activity is therefore broader than simply controlling protein synthesis.
41. mTORC1 vs mTORC2
| Feature | mTORC1 | mTORC2 |
|---|---|---|
| Major function | Growth/anabolism | Survival/cytoskeleton/AKT regulation |
| Key downstream target | S6K | AKT |
| 4E-BP1 | Yes | No major role |
| Nutrient sensitivity | Strong | Less direct |
| Autophagy | Suppresses | Indirect effects |
| Protein synthesis | Strongly promotes | Indirect |
| AKT | Downstream target through feedback | Direct regulatory target |
42. Major mTORC1 Components
mTORC1 contains:
- mTOR
- Raptor
- mLST8
- Regulatory proteins including PRAS40 and DEPTOR
The composition helps determine substrate recruitment and pathway regulation.
43. Major mTORC2 Components
mTORC2 contains:
- mTOR
- Rictor
- mSIN1
- mLST8
- Other regulatory components
Important distinction
mTORC1 → Raptor
mTORC2 → Rictor
This is a common examination question.
44. PI3K–AKT–mTOR and Cancer
The pathway is frequently dysregulated in cancer.
Possible mechanisms include:
- PI3K activating mutations
- PTEN loss
- AKT activation
- RTK amplification
- mTOR dysregulation
PI3K activation
↓
↑ PIP3
↓
↑ AKT
↓
↑ mTOR
↓
Growth + survival
↓
Tumor progression45. PI3K Mutations
An important example is activating alterations in:
PIK3CA
PIK3CA encodes the p110α catalytic subunit of class IA PI3K.
Activating mutations can result in increased PI3K–AKT signaling.
46. PTEN Loss in Cancer
Loss of PTEN produces:
PTEN loss
↓
PIP3 accumulation
↓
AKT activation
↓
mTOR signaling
↓
Increased survival and growthThus PTEN loss can mimic excessive upstream PI3K activation.
47. Therapeutic Targeting
The pathway provides several pharmacological targets:
PI3K inhibitors
Target PI3K activity.
AKT inhibitors
Target AKT.
mTOR inhibitors
Target mTOR signaling.
Some agents preferentially inhibit mTORC1, while newer approaches can inhibit both mTOR complexes depending on mechanism.
48. Rapamycin and mTOR
Rapamycin and related drugs are important experimental and clinical tools for studying mTOR signaling.
Rapamycin primarily inhibits mTORC1 through an FKBP12-dependent mechanism, although the exact effects can vary with exposure and cellular context.
49. Feedback Regulation
PI3K–AKT–mTOR signaling contains extensive feedback loops.
One important mechanism involves mTORC1/S6K-mediated feedback that can reduce signaling through upstream insulin receptor substrate proteins.
Conceptually:
PI3K
↓
AKT
↓
mTORC1
↓
S6K
↓
Negative feedback
↓
Reduced upstream signalingThis prevents unlimited pathway activation.
50. Crosstalk with Ras–MAPK
Growth-factor receptors can simultaneously activate:
RTK
│
┌──────────┴──────────┐
↓ ↓
RAS–MAPK PI3K–AKT
↓ ↓
Proliferation Survival
Differentiation GrowthThese pathways frequently cooperate.
51. Integrated Growth-Factor Response
A growth factor can therefore produce:
GROWTH FACTOR
↓
RTK
↓
┌──────────────┴──────────────┐
↓ ↓
RAS–MAPK PI3K–AKT
↓ ↓
Gene expression mTORC1
↓ ↓
Proliferation Protein synthesis
↓
Cell growthThis explains why cancer cells often require simultaneous dysregulation of multiple signaling pathways.
52. Systems-Biology View
The pathway can be conceptualized as an input integration system:
GROWTH FACTORS
↓
RTK
↓
PI3K
↓
PIP3
↓
AKT
↓
┌─────┴─────┐
↓ ↓
mTORC1 FOXO
↓ ↓
Anabolism Survival
↓
Cell growth
↑
│
Nutrient sensing
│
Energy sensingThe cell therefore integrates extracellular and intracellular information before committing to growth.
53. High-Yield Molecular Relationships
| Molecule | Main function |
|---|---|
| PI3K | Generates PIP3 |
| PIP3 | Membrane signaling lipid |
| AKT | Major serine/threonine kinase |
| PDK1 | AKT activation |
| mTORC1 | Growth and anabolic metabolism |
| mTORC2 | AKT regulation and cytoskeleton |
| TSC1/TSC2 | Negative regulator of mTORC1 |
| RHEB | Activator of mTORC1 |
| PTEN | Converts PIP3 toward PIP2 |
| S6K | Promotes translational growth |
| 4E-BP1 | Translation initiation regulator |
| FOXO | Transcription factor regulated by AKT |
| AMPK | Energy-stress regulator; inhibits mTORC1 |
54. Important Molecular Switches
There are several different types of switches in this pathway.
Lipid switch
PIP2 ↔ PIP3
controlled by:
- PI3K
- PTEN
Protein kinase switch
AKT activation
controlled by phosphorylation.
Small GTPase switch
RHEB-GDP ↔ RHEB-GTP
regulates mTORC1.
This illustrates the multilayered regulation of the pathway.
55. Master-Level Integrated Diagram
GROWTH FACTOR
↓
RTK
↓
PI3K
↓
PIP2 ───→ PIP3
↑ ↓
│ AKT
PTEN │
↓
┌───────────┴───────────┐
↓ ↓
FOXO TSC1/TSC2
↓ ↓
Gene regulation RHEB-GTP
↓
mTORC1
↓
┌──────────────┴──────────────┐
↓ ↓
S6K 4E-BP1
↓ ↓
Translation eIF4E
└──────────────┬──────────────┘
↓
PROTEIN SYNTHESIS
↓
CELL GROWTH
Nutrients ──→ Rag GTPases ──→ mTORC1
Low energy ──→ AMPK ──| mTORC1
mTORC2 ──→ AKT regulation56. Comparison with Ras–MAPK
| Feature | PI3K–AKT–mTOR | Ras–MAPK |
|---|---|---|
| Major lipid intermediate | PIP3 | None |
| Major GTPase | RHEB | RAS |
| Major kinase | AKT/mTOR | RAF/MEK/ERK |
| Major output | Growth, survival, metabolism | Proliferation, differentiation |
| Major negative regulator | PTEN | RAS-GAPs |
| Major transcriptional effect | FOXO and growth programs | ERK-regulated transcription |
| Major metabolic role | Very strong | More indirect |
| Autophagy | Strong regulation through mTORC1 | Less central |
57. Examination Answer
PI3K–AKT–mTOR Pathway
The PI3K–AKT–mTOR pathway is a major intracellular signaling pathway that regulates cell growth, survival, metabolism, protein synthesis and autophagy. It is commonly activated downstream of receptor tyrosine kinases and the insulin receptor.
Ligand binding activates the receptor, leading to recruitment and activation of class I PI3K. PI3K phosphorylates membrane PIP2 to generate PIP3. PIP3 recruits proteins containing PH domains, including AKT and PDK1, to the plasma membrane. AKT is then activated through phosphorylation involving PDK1 and mTORC2.
Activated AKT promotes mTORC1 activity partly by inhibiting the TSC1/TSC2 complex, thereby allowing RHEB-GTP to activate mTORC1. mTORC1 phosphorylates S6K and 4E-BP1, promoting protein synthesis and cellular growth. AKT also promotes cell survival through regulation of targets such as FOXO.
The pathway is negatively regulated by PTEN, which reduces PIP3 levels, and by AMPK under conditions of low cellular energy. mTORC1 integrates growth-factor, nutrient and energy signals and suppresses autophagy when nutrients are abundant.
Dysregulation of PI3K–AKT–mTOR signaling, including PIK3CA activation, PTEN loss and AKT/mTOR pathway activation, is common in cancer.
58. High-Yield Viva Questions
Q1. What does PI3K stand for?
Phosphoinositide 3-kinase.
Q2. What is the major lipid product of PI3K?
PIP3.
Q3. What does PTEN do?
It dephosphorylates PIP3 and thereby antagonizes PI3K signaling.
Q4. What is AKT?
A serine/threonine protein kinase, also called protein kinase B.
Q5. How is AKT recruited to the membrane?
Through its PH domain binding to PIP3.
Q6. Which kinase phosphorylates AKT downstream of PIP3?
PDK1 contributes to AKT activation, while mTORC2 provides another key activating phosphorylation.
Q7. Name the two major mTOR complexes.
mTORC1 and mTORC2.
Q8. What is the major function of mTORC1?
Regulation of cellular growth and anabolic metabolism, especially protein synthesis.
Q9. What is the major function of mTORC2?
Regulation of AKT and other AGC kinases, cell survival and cytoskeletal organization.
Q10. What is the major downstream target of mTORC1 involved in translation?
S6K and 4E-BP1.
Q11. What is the role of 4E-BP1?
It regulates eIF4E and translation initiation.
Q12. What is the TSC1/TSC2 complex?
A negative regulator of mTORC1 that controls RHEB.
Q13. What activates mTORC1 downstream of TSC inhibition?
RHEB-GTP.
Q14. Which enzyme inhibits PI3K signaling by reducing PIP3?
PTEN.
Q15. What happens to mTORC1 during energy stress?
AMPK activation generally suppresses mTORC1.
Q16. What happens to autophagy when mTORC1 activity decreases?
Autophagy generally increases.
Q17. Name an important oncogenic PI3K alteration.
Activating alteration of PIK3CA.
Q18. Why is PTEN considered a tumor suppressor?
Because it restrains PIP3–AKT signaling.
59. One-Minute Revision
GROWTH FACTOR
↓
RTK
↓
PI3K
↓
PIP2 → PIP3
↓
AKT
↓
┌─────────┴─────────┐
↓ ↓
Survival TSC1/TSC2
↓ ↓
FOXO RHEB-GTP
↓
mTORC1
↓
┌────────────┴────────────┐
↓ ↓
S6K 4E-BP1
↓ ↓
Translation eIF4E
└───────────┬─────────────┘
↓
PROTEIN SYNTHESIS
↓
CELL GROWTH
PTEN ──────────────| PIP3
AMPK ──────────────| mTORC1
mTORC2 ───────────→ AKT regulationCore memory line
RTK → PI3K → PIP3 → AKT → TSC inhibition → RHEB → mTORC1 → S6K/4E-BP1 → protein synthesis → cell growth
Three essential concepts
PI3K makes PIP3.
PTEN removes PIP3.
mTORC1 integrates growth-factor, nutrient and energy signals to control cellular growth and anabolism.